There exists a vital and continuing need for wireless communication networks of various types. Certain particular wireless systems are focused on the need for reliable two-way data communications. Such networks need not support particularly high data exchange rates, but should provide communication over as wide a geographic area as possible, such as the continental United States or Europe.
Unfortunately, many existing and even certain proposed systems costing millions of dollars have failings of one type or another. Consider, for example, existing wireless wide area data networks which support communication between a remote or mobile field unit and a base station. These networks either use terrestrial or satellite-deployed base stations. Terrestrial systems can be further classified as either one-way or two-way. One way terrestrial systems, such as nationwide paging networks like SkyTel, provide no capability for a remote user to send data. Although certain types of paging networks do support two-way data transfer, they only provide limited geographic coverage. In addition, such networks also typically exhibit relatively poor penetration of building structures, due to the high carrier frequencies at which they operate.
Other existing and proposed two-way terrestrial systems include the cellular networks, mobile data networks such as RAM, ARDIS, emerging PCS networks, EMBARC, and many others. While the data rates of these systems are typically quite high, each system requires the users to be within a close range, generally 20 miles or less, of the system base station infrastructure. This infrastructure is extremely expensive, requiring hundreds of millions of dollars to build a nationwide network. It can sometimes be cost effective to build such infrastructure in areas of high population density, and indeed, roughly 90% of the United States population can be supported by such systems. However, this terrestrial infrastructure only covers approximately 15-20% of the country geographically. It is simply not economical for providers of such services to install the required infrastructure in remote areas of low population density.
Several satellite networks, both existing and proposed, have been designed to address the issue of poor geographic coverage. These satellite-based systems also typically require a tremendous investment in infrastructure. The infrastructure is located in orbit where it cannot be installed, maintained or replaced without large expenditures for space-launch vehicles. In addition, the mobile subscriber or field devices required to communicate with such satellite systems are relatively expensive. Furthermore, the field devices need to be within the line of sight of the satellite, since they must typically have overt, high gain electromagnetic reception devices such as dishes or long antennas. These systems, too, are thus impractical for certain applications.
Consider the set of problems faced by the manager of a fleet of rental cars. The assets for which the manager is responsible are highly mobile--indeed, they can be located virtually anywhere in the continental United States. The assets are also easily stolen and thus expensive to insure. They can become unproductive when a rental customer fails to return a vehicle to its proper location. Rental cars can also become `lost` when there is poor communication between retail outlets, and valuable up-time of the rental asset is then squandered.
Another issue important to managers of rental fleets is the safety of their customers. Rental car drivers, and in fact, all drivers, could benefit from a system would summon emergency assistance at any time, from any location, without leaving the vehicle.
Analogous problems existing in other industries. For example, there is increasing pressure on the railroad industry to move towards scheduled service, thereby facilitating just-in-time delivery, in an effort to better compete with the trucking industry. To achieve this goal, the manager of a railroad system would ideally be able to quickly determine the location of each and every rail car on a regular basis, no matter where the rail car is located. Optimum routing and delivery time could then be accurately predicted.
In both of these fleet management applications, the fleet manager would very much like to be able to query a remote device, in order to determine its location, but at minimum cost. Existing systems do not fulfill this need--for example, current cellular telephone service carries with it relatively high connect time charges, roaming charges, and monthly service fees, and fleet managers do not consider such systems to be cost effective.
Other industries, such as the trucking and shipping industries, could also benefit from the ability to inexpensively and accurately track the location of shipping containers no matter where they are located. Any one shipping container may hold thousands or potentially millions of dollars of valuable goods, and clearly, those responsible for the well being of the goods in transit would like to know where they are at all times.
Similar demands are made in remote meter or sensor reading, facility monitoring, security, buoy monitoring, and other applications.
One way to provide low cost, long haul communications service is by using short wave radio links that operate in the High Frequency (HF) radio band, which ranges from approximately 3 to 30 MegaHertz (MHz). Radios which operate in this band have been in use for many years, and the required transceiver equipment is inexpensive to maintain and operate. Signals transmitted at HF frequencies can be carried for hundreds or even thousands of miles. However, there are certain well-known difficulties which make HF radio transmission unreliable. A first problem is rooted in the fact that HF provides long distance, over the horizon communication by bouncing the signal off of the earth's ionosphere. Due to multiple atmospheric conditions, a phenomena which changes depending upon location, time of day, time of year, and sun spot activity levels, different portions of the 3-30 MHz spectrum may or may not propagate in different directions at any given time of day. Thus, in order to provide reliable communication, the transmitting radio must make some accomodation for the fact that a chosen carrier frequency in the HF band may or may not be propagating between itself and the receiver.
Secondly, of those frequencies which are propagating, the transmitter and receiver must know also which frequencies are clear, that is, which frequencies are not in use by other equipment operating in the same band. This problem is not as easy to solve as it may seem. While certain frequencies in the HF spectrum are dedicated in advance to certain known users, many other frequencies in the HF band remain available for on-demand use. Thus, it cannot be predicted with certainty when these frequencies will or will not be occupied at any instant in time.
Traditionally, HF communication systems have depended upon trial and error to find a frequency which is both propagating and clear. These systems thus only provide minimum reliability in terms of the probability of establishing a link from the transmitter to receiver exactly when that link is desired.
More advanced systems improve reliability by using "sounder" techniques together with automatic link establishment (ALE) algorithms. In those systems, the base station transmits on multiple frequencies, and the remote receivers listen on the same multiple frequencies. When the remote hears the base station, it knows that the frequency that is heard was propagating. The remote then transmits on that frequency as soon as the base completes its transmission, before the frequency can be occupied by another user.
Unfortunately, even ALE-type systems have several drawbacks. First, they are spectrally inefficient, since the base stations must broadcast on several frequencies. Second, the remote units are more expensive than would otherwise be required, because they need to contain frequency agile HF receivers as well as an HF transmitter. The system capacity, in terms of how many remote units can be supported, is limited because of the need to transmit on multiple frequencies at the same time.
Finally because a single central HF base station coordinates the use of the outbound links, the geographic coverage of such a system is limited to that which can be provided by a single base station and reliability is minimized if that base station is not in a region that is condusive to propagation.